Various forms of four-wheel drive vehicles have been proposed and many of them have already been put to actual uses. They can be generally grouped into those which are called as full-time four-wheel drive vehicles and those which are called as part-time four-wheel drive vehicles. In a full-time four-wheel drive vehicle, both the front wheels and the rear wheels are always power driven, typically by way of a central mechanical differential device. Alternatively, a viscous coupling which transmits torque substantially in proportion to the difference in the rotational speeds of its input member and output member may be interposed in the path of power transmission between the front wheel set and the rear wheel set in order to achieve a four-wheel drive. On the other hand, in a part-time four-wheel drive vehicle, either the front wheels or the rear wheels are normally not power driven at all but all the wheels are power driven typically by rigidly connecting the front wheel set and the rear wheel set together only when so selected by the driver. In some of the full-time four-wheel drive vehicles, the central differential device is provided with a lock mechanism for selective rigid locking up of the differential device to improve the capability of the vehicle to run on a frozen, muddy or otherwise slippery road surface.
In either case, it could happen for the front wheel set and the rear wheel set to be rigidly connected to each other. When that is the case, the braking force applied to one of the wheel sets is directly transmitted to the other wheel set. However, it is more desirable to have the front wheel set experience a greater braking force than the rear wheel set as is the case in most four-wheel vehicles for a balanced braking action and the rigid connection between the front wheel set and the rear wheel set causes the braking force to be evenly distributed between the front wheel set and the rear wheel set.
In order to eliminate this problem, Japanese Utility Model Laid-Open Publication No. 59-188731 (corresponding to German Patent Application P No. 33 17 247.1 and U.S. Pat. No. 4,605,087) proposed provision of a freewheel device in the path of power transmission between the front wheel set and the rear wheel set to the end of blocking the transmission of braking force from the front wheel set to the rear wheel set. However, according to this proposal, the braking force of the engine brake is also not transmitted to the rear wheel set. Thus, the advantage of four-wheel drive will be lost for instance when the vehicle goes down a slippery slope, since the braking force of the engine brake will not be transmitted to the rear wheels. Furthermore, when the vehicle goes backward, power will not be transmitted to the rear wheel set and, therefore, it will be necessary to provide a manual lock mechanism for locking up the freewheel device in view of the possibility of advancing backward in a muddy or otherwise slippery terrain.